Critical Conditions and the Choking Mass Flow Rate in Nonequilibrium Wet Steam Flows

A theoretical analysis of choking in steady, one-dimensional, nonequilibrium, wet steam flows is presented. It is shown that such a flow becomes choked when the vapor phase velocity attains the frozen speed of sound somewhere in the system. The upstream flow pattern cannot then be altered by small adjustments of the back pressure and the mass flow rate is close to, although not necessarily identical to, its maximum value. The equilibrium speed of sound has no physical relevance in such flows. In a choked converging nozzle the critical conditions always occur in the exit plane of the nozzle. In a converging-diverging nozzle, however, the shape of the diverging section influences the throat conditions and throughput. Comparison of the theory with the few experiments reported in the literature shows excellent agreement.